Performance monitoring for loops

ABSTRACT

Loop Performance Monitoring (LPM) for DDS loops is described. Even though DDS loops have Intentional Bipolar Violations (BPVs), a Loop Coding Violations (LCVs) detection strategy based on further processing of BPVs is described. By monitoring LCVs a local loop terminating device can determine Bit Error Rate (BER). 
     A system is described by which an Office Channel Unit (OCU) can process LCV information to determine signal quality of the signal over the incoming local loop. If the signal quality falls below a certain threshold, the OCU can cut the loop off from the DDS circuit and send control codes into the network. 
     A system is also described where a Network Interface Unit (NIU) with the LPM system communicates incoming LCV information to the OCU using low speed signalling over the simplex path between the transmit and receive pairs. The OCU monitors incoming LCVs as well, and thus has the information necessary to determine bi-directional BER performance.

BACKGROUND OF THE INVENTION

DDS Loop Technology and Coding Format

Dataphone Digital Service (DDS) is a private line digital datapoint-to-point, or point-to-multipoint communication service, widelydeployed in the United States of America and is described in detail inthe following references (incorporated herein by reference):

1, "D4 Digital Channel Bank Family", Bell System Technical Journal, Vol.61, Number 9, Part 3, November 1982.

2. "Digital Data System", Bell System Technical Journal, Vol. 54, Number5, May--June 1975.

3. "Generic Requirements for the Subrate Multiplexer", Issue 1TA-TSY-000189, Bell

Communications Research Inc., April 1986.

The data rate at which a DDS customer can obtain service are thesubrates 2.4 kbps, 4.8 kbps, 9.6 kbps, 19.2 kbps, 38.4 kbps and the fullrates 56 kbps, 64 kbps. The local distribution of a DDS connection usesmetallic, twisted-pair cables for the full-duplex four-wire transmissionpath between the customer premises and the serving DDS office. A ChannelService Unit (CSU) serves to terminate the four-wire loop at thecustomer premises. At the DDS serving office the loop is terminated withan Office Channel Unit (OCU). The OCU encodes the incoming data signalsinto an 8-bit byte format that adds necessary control information and,regardless of the data service rate, builds the signal up to a rate of64 kbps.

The DDS local loop signalling format employs Alternate Mark Inversion(AMI) encoding to convert digital data signals generated at the customerpremises to an AMI line code format. In AMI, a ONE bit is transmitted asa pulse transition (polarity change) and a ZERO bit as no pulse. Thedigital signal on the local loop is baseband, bipolar, return-to-zero,with 50-percent duty cycle. This signal has a symbol rate equal to thedata rate. A binary 1 is transmitted as a positive or negative pulse, inopposite polarity to the preceding pulse. A binary 0 is represented bythe absence of a pulse. The local loop signal format is described indetail in the following reference (incorporated herein by reference):

4. "Digital Data System Requirements for the Office Channel Unit", Issue1, TA-TSY-000083, Bell Communications Research Inc., December 1984.

As shown in FIGS. 1a and 1b, the loop signal between the CPE 10 and theOCU 12 (with non-secondary channel information) is formatted in 6 bitbytes containing six channel data bits D1-D6 for the data rates of 2.4,4.8, 9.6, 19.2 and 38.4 kpbs (FIG. 1a), or 7 bit bytes containing sevenchannel data bits D1-D7 for 56 kbps (FIG. 1b). The presence of networkcontrol information is indicated by modifying the standard bipolarsignal. The signal is modified by inserting a violation `V` pulse intothe bit stream. This pulse has the same polarity as the immediatelypreceding pulse (hereinafter "the "previous" pulse), thus violating thestandard format.

Unrestricted insertion of violations would produce an undesirable dccomponent on the local loop. Therefore, a bit period is reserved twobits prior to a violation, for insertion of a bipolar pulse or no pulse,i.e., a zero, in such a manner that successive violations alternate inpolarity. Calling this inserted pulse an `X` bit; modified bipolarsignals are shown in FIGS. 2a and 2b with X bit values of 0 and 1. Ifthe number of pulses since the previous violation is odd, the X bit is azero. If the number of pulses since the last violation is even, a pulseof opposite polarity to the previous pulse is inserted into the pulsestream as the X bit. The complete violation sequence includes a forcedzero between the X and V bits; therefore, the sequence is called an XOVsequence.

A secondary channel capability has been proposed to offer a companiondigital transmission channel independent of the primary channel and at alower rate. Secondary channel capability requires that the loop signalbe structured so that the primary and secondary channel information canbe differentiated. As shown in FIGS. 3a and 3b, the loop signal withsecondary channel information is formatted in 8 bit bytes containing sixprimary channel data (D) bits D1-D6 for the primary channel rates of2.4, 4.8, 9.6, 19.2 and 38.4 kbps (FIG. 3a), or 9 bit bytes containing 7D bits for 56 kbps primary channel data, or 9 bit bytes containing 8 Dbits for 64 kbps primary channel data (FIG. 3b). Each byte contains an"F" bit for framing, and a "C" bit arising out of the substitution ofthe secondary channel information on no more than one out of every threeC bits (FIG. 3c). Intentional Bipolar Violations (BPV) in the loopsignal with secondary channel information are not required. BPVs areused in the basic DDS service to transmit control and supervisoryinformation.

MJU Circuits and Streaming Branches

The DDS provides full duplex, synchronous, end-to-end digitaltransmission on dedicated private line two point and multipointcircuits. A two point circuit connects two customer stations. Amultipoint circuit allows several customer stations to share a commoncommunication channel using Multipoint Junction Units (MJU) located inDDS offices. The customers control station broadcasts downstream to oneor more remote stations. In the upstream direction the MJUs combine thebit streams transmitted by the remote stations into a serial bit streamfor delivery to the control station. The DDS multipoint service servesonly those multipoint circuits that have a single customer-controllocation and a number of remote stations. The duplex data path from theMJU to the customer's control station (upstream direction) is called thecontrol channel while the duplex data paths from an MJU toward theremote stations (downstream direction) are called branches. It is theresponsibility of the data customer to use an appropriate "pollingtechnique" so that only one remote station is transmitting data towardthe control station at any given time.

In the upstream direction, the MJU receives a steady stream of theControl Mode Idle (CMI) code (S1111110) or Data Mode Idle (DMI) code(S1111111) when the customer is not transmitting data on a branch. Whena remote station becomes active, the idle condition will change to adata pattern provided by the customer. Functionally, the MJU can "AND"the data bits and "OR" the control bits from the branches. Note that ifmore than one branch is carrying an active data signal, the controlchannel output will be the garbled combination of the branch inputsignals. Since many control conditions can be signaled upstream, allnetwork control codes (bit 8=0) received on any branch are treated as ifthey were the same as CMI/DMI for purposes of forming the upstreamtransmission byte. The MJU operation is described in detail in thefollowing reference (incorporated herein by reference):

5. "Digital Data System--Multipoint Junction Unit Requirements", Issue2, TA-TSY-000192, Bell Communications Research Inc., April 1986.

On a multipoint circuit, one or more idle noisy station loops (orbranches), which are experiencing errors (called streaming branches),interfere with a customer's active branch. In other words, streamingbranches could cause total multipoint network failures. Thus, theoverall reliability of multipoint circuits worsens as the number ofbranches increases.

A need exits, therefore, for a system for automatically identifyingnoisy branches and disconnecting them from the multipoint circuit.

Proactive Maintenance

DDS is designed to be a high performance service. A typical DDS circuitterminates at the OCU at the End Office and is cross connected toT-carrier facilities for inter-office or inter-LATA haul. T-carrier usesan AMI line format and error performance of these T-carrier facilitieshas traditionally been monitored using BPVs. As the error performance ofthese facilities begins to degrade, preventive maintenance is triggered,often before the resulting performance is unacceptable given DDSperformance requirements. If enough DDS circuits are present in thecarrier, automatic protection switching is employed on the facility.

Unfortunately, DDS local loops are the most vulnerable part of thecircuit. Since, for many of the DDS rates, BPVs are deliberatelyemployed to transport control conditions over the local loop, heretoforeit has not been possible to employ similar techniques as T-carrier forthese loops.

Service capability, equivalent to DDS, multiplexed with other services(like voice frequency (VF) services) can be provided using T1 facilitiesdirectly to the customer's premises, or by local bypass serviceproviders (principally Inter Exchange Carriers (IXCs)). This, of course,results in revenue loss for the telephone company, loss of accountcontrol, and does nothing to solve the problem for the small servicesites where there are no other service needs to multiplex with the datacommunication needs. To further differentiate their service, IXC'semploy an ESF format over the access T-carrier facility. In ESF format,part of the framing bit bandwidth carries Cyclic Redundancy Check (CRC)information which allows measurement of Bit Error Rate (BER). In theframing bit bandwidth, ESF also carries a full duplex Facilities DataLink (FDL) which, amongst other information, is used to relay backincoming BER information on the outgoing carrier facility at thecustomer's premise. Thus, bi-directional performance information isavailable at the central office to trigger an appropriate proactivemaintenance strategy. The ESF framing format is described in detail inthe following reference (incorporated herein by reference):

6. "Carrier to Customer Installation--DS1 Metallic Interface" ANSI T1,403-1989.

A need exists, therefore, for an equivalent proactive maintenance systemfor DDS local loops.

SUMMARY OF THE INVENTION

An apparatus and method is provided for automatically detecting BPV'soccurring in a two-wire local loop digital AMI encoded baseband bipolarreturn-to-zero signals transmitted as two trains of pulses ReceivePositive Rail (RPR) pulses and Receive Negative Rail (RNR) pulses on arespective rail (wire) of the local loop. The signals on each rail aresynchronized. Present RPR pulses are AND'ed in an AND gate with previousRPR pulses. A BPV is detected when both the present and previous RPRpulses are positive. The previous RPR pulse is stored when a number ofzeros occur between two positive RPR pulses and the previous positivepulse is eventually AND'ed with the present positive pulse to detect aBPV. If the present pulse on the positive rail is negative the previouspositive pulse is no longer stored. Instead, a zero level pulse iscoupled to the AND gate to be AND'ed with the present pulse (which isnegative) in which case the output of the AND gate will be a zero and noBPV will be indicated.

The signals on the Receive Negative Rail are processed in a similarfashion in a negative BPV circuit and the output of the AND gate of thenegative BPV circuit is OR'ed with the output of the AND gate of thepositive BPV circuit to produce an automatic indication of a BPVoccurrence on either rail.

An apparatus and method is also provided for detecting XOV's bydetermining the occurrence of a BPV having the same polarity as the lastBPV. In this system the present positive pulse and the previous positivepulse on the positive rail are AND'ed with a pulse that only becomeshigh when the previous pulse on the positive rail is a zero level pulse.A positive XOV is detected when the present positive pulse is a BPV andthe previous pulse is a zero level pulse. Consecutive zero level pulsesbetween two positive pulses are ignored as before by holding theprevious positive pulse in storage. Negative XOV's are detected in asimilar fashion.

In addition an apparatus and method is provided for preventing noisybranch stations, which are causing errors, and thus possibly interferingwith a customer's active branch. This is accomplished by monitoring thedata received from the loop for streaming errors, i.e., unintentionalBPV's. When a streaming error is detected an Abnormal Status Code (ASC)comprising a bit data byte of S0011110. In accordance with standard DDSmultipoint bridging rules the ASC code coming in on a branch signals ortells the multipoint bridge to not include that branch on the bridge.

An equivalent proactive maintenance technique for DDS local loops isprovided which uses a Network Interface Unit (NIU) bridged on to thelocal loop at the customers premise to monitor the incoming loop forloop code violation (LCVs). The NIU signals back the presence of LCVs tothe OCU at the Central Office by creating a slow speed data link on thesimplex path between the incoming and the outgoing local loop. The OCUcan thus collect bi-directional performance information and provide itto a proactive maintenance application instrumented by the telephonecompany.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a schematic representation of the loop byte format for loopsignalling between OCU's and CPE's at subrate speeds.

FIG. 1b is a schematic as in FIG. 1a for 56 Kb/s signally.

FIG. 2a is plot of amplitude versus time o one byte of the subrate speeddata signal in which network control information is indicated bymodifying the standard bipolar signal.

FIG. 2b is a plot as in FIG. 2a for the 56 Kb/s signal.

FIG. 3a is a schematic representation of the loop byte format forsubrate speeds loop signalling which includes a secondary channelcapability.

FIG. 3b is a schematic as in FIG. 3a for 56 Kb/s signalling.

FIG. 3c is a table explaining the bit format of FIGS. 3a and 3b.

FIG. 4 is a schematic diagram of a BPV detection circuit.

FIG. 5 is a timing diagram illustrating the details of each of thesignals in FIG. 4.

FIG. 6 is a schematic diagram of an XOV detection circuit.

FIG. 7 is a timing diagram illustrating the details of each of thesignals in FIG. 6.

FIG. 8A and 8B are a schematic diagram of a non-alternating XOVdetection circuit.

FIG. 9 is a timing-diagram illustrating the details of each of thesignals in FIG. 8.

FIG. 10 is a schematic diagram of an NIU and OCU sealing current pulsedetection circuit.

DETAILED DESCRIPTION OF THE INVENTION

Monitoring Loop Errors Through Loop Coding Violations (LCVs)

The reliability problems associated with multipoint circuits describedabove can be solved if a mechanism can be provided to automaticallyidentify noisy branches and cutting them off from the multipointcircuit. Since, for many DDS rates, BPVs are intentionally present on aDDS loop, BPVs alone are not an adequate measurement of loopperformance. The following is a detailed description of a system fordetermining the occurrence of LCVs. The frequency of occurrence of LCV'sis a measure of DDS loop performance. An OCU is also described that upondetermining that the terminated loop is noisy by the frequency ofoccurrence of LCV's, transmits network control codes upstream into themultipoint circuit. This feature allows the service provider to deliverthe same reliability performance on multipoint circuits as private linecircuits.

Monitoring for LCV's in Secondary Channel and Clear--64kbps rates

The Secondary Channel (SC) capability in the DDS network offers acompanion lower bit rate digital transmission channel that isessentially independent of the primary DDS channel. It is implemented bytime-sharing the network control bit (bit 8) position with the customeras discussed below. As previously described in connection with FIGS. 3a,b, c; customer's data stream on the loop is formatted in 8 bit bytescontaining 6 primary channel rates of 2.4, 4.8, 9.6, 19.2 and 38.4 kbps,and 9 bit bytes containing 8 D bits and 1 F bit for clear--64 kbpschannel data. In SC loop format, network control and supervisoryinformation is encoded into C' bit so that intentional BPVs are notnecessary. In clear--64 kbps loop format, network control andsupervisory information is transmitted using a number of data sequences,not using the C' bit, so that intentional BPV is not necessary.Therefore, any signal BPV on the local loop can be interpreted as anerror event. Thus, for these rates, LCVs and BPVs are the same.

A BPV detection circuit in accordance with the invention will now bedescribed, in detail, in connection with FIG. 4. As shown in FIG. 4, theReceive Positive Rail (RPR) pulses and the Receive Negative Rail (RNR)pulses are the identical inverse of one another and are separatelycoupled to respective flip-flop circuits FF1 and FF2 where the two railsare retimed by Receive Loop Rate Clock pulses (RLRCLK) from a source(not shown) of clock pulses. The Q Output of FF1 is the present positiverail pulse and the Q output of FF3 is the previous positive rail pulsetrain, i.e., the previous pulse train which has been stored and delayedby one clock pulse period. These two positive rail pulses are coupled toan AND gate AND1. The output of AND1 goes positive only when twopositive pulses are simultaneously present at its two input terminals.Positive rail BPV is detected by the positive pulse from AND1 when bothof the present and previous positive rail pulses are positive at thesame time. The multiplexer MUX1 holds the previous positive rail pulseuntil the next pulse shows up. So, in the case of a number of zerosbetween two positive pulses, the MUX1 will hold the previous positivepulse during a number of zeros by looping the Q output of FF3 back tothe D input of FF3. The present positive pulse will eventually becoupled to AND1 and positive BPV will be detected. If the present pulseis negative, then the feedback loop on FF3 is broken and a zero levelpulse is coupled to the D input of FF3 and no BPV is detected. Thenegative BPV detection circuit consists of FF2, MUX2, FF4 and AND2, andfunctions exactly same as the positive BPV detection circuit. The outputof OR gate OR2 is an active high positive/negative BPV signal which iscoupled to FF5 and retimed by RLRCLK. The timing diagram of FIG. 5illustrates the details of each signals where "B" denotes a bipolarpulse signal and "0" denotes a zero level signal and "V" denotes aviolation pulse signal.

For subrates and 56 kbps rates

The DSO cross-connect format between OCUs and multiplexing equipment isbased on 8 bit bytes as follows: F1 D2 D3 D4 D5 D6 D7 C8

Bit F1 is used either for the subrate multiplexer framing code or fordata in the case of 56 kbps service. Bits D2 through D7 are used fordata in all services. Bit C8 is dedicated as the network control modeidentifier. When bit C8 is a 1, bits D2 (or F1 for 56 kbps service)through D7 are identified as customer data. When bit C8 is a 0, bits D2through D7 are interpreted as network control information. As describedabove, a baseband bipolar signal is used for transmission between OCUsand CSUs over local loops. When bit C8 is a 1, the normal bipolar ruleapplies, and the resultant line signal carries no bipolar violationpulse. When bit C8 is a 0, a bipolar violation encoding rule is applied,and the resultant line signal carries XOV bipolar violation sequence.The violation pulse V uniquely establishes the network control mode andalso identifies the byte alignment. The system determined pulse X is setto force the number of B pulses between violations to be odd. Thiscauses successive violations to alternate in sign, thus limiting dcbuild-up in the transmitted signal. Therefore, an XOV bipolar violationsequence which has opposite polarity as a preceding XOV bipolarviolation sequence should not be interpreted as an error event. However,any BPV which does not have an XOV bipolar violation sequence should beinterpreted as an error event. Any XOV bipolar violation sequence whichhas the same polarity as a preceding XOV bipolar violation sequenceshould also be interpreted as an error event. Thus, for these rates anLCV is the occurrence of a BPV which has the same polarity as the lastBPV.

The XOV detection circuit will now be described, in detail, inconnection with FIG. 6. As shown in FIG. 6, the Receive Positive rail(RPR) pulses and the Receive Negative Rail (RNR) pulses are separatelycoupled to flip-flop circuits FF1 and FF2 where the two rails areretimed by clock pulses Receive Loop Rate Clock (RLRCLK). The Q outputof FF1 is the present positive rail pulse and the Q output of FF3 is theprevious positive rail pulse. The Q output of FF5 is the OR of theprevious positive rail pulse and the previous negative rail pulse and iscoupled to an inverter INV1. So, the output of INV1 becomes active highonly when the previous pulse is a zero level pulse. These three outputs;FF1, FF3 and INV1, are coupled to an AND gate AND 1 where positive XOVis detected when the present positive rail pulse is a bipolar violationpulse and the previous pulse is a zero level pulse. Thus, the output ofAND 1 is an active high positive XOV bipolar violation signal. Themultiplexer MUX1 holds the previous positive pulse until the next pulseshows up. So, in the case of a number of zeros between two positivepulses, the MUX1 will hold the previous positive pulse during a numberof zeros by looping the Q output of FF3 back to the D input of FF3. Thenegative XOV detection circuit consists of FF2, MUX2, FF4, FF5, INV1 andAND2, and functions exactly same as the positive XOV detection circuit.The output of OR gate OR2 is an active high positive/negative XOV signalwhich is coupled to FF6 and retimed by RLRCLK. The timing diagram ofFIG. 7 illustrates the details of each signal in FIG. 6 where "B"denotes a bipolar pulse signal and "0" denotes a zero level signal and"V" denotes a violation pulse signal and "X" denotes a system-determinedpulse signal that may be either a "0" or a "B".

The non-alternating XOV detection circuit will now be described, indetail, in connection with FIGS. 8A and 8B. As shown in FIG. 8A and 8B,the non-alternating XOV detection circuit 18 consists of a XOV detectioncircuit 16 as a first stage and a BPV detection circuit 14 as a secondstage. The first stage of XOV detection circuit detects either apositive XOV or a negative XOV. These two XOV detection signals fromAND1 and AND2 are separately coupled to the second stage, BPV detectioncircuit 14. The output of AND1 is the present positive XOV pulse and theQ output of FF6 is the previous positive XOV pulse. These two positiveXOV pulses are coupled to an AND gate AND3 where positivenon-alternating XOV is detected when both of the present and previouspositive XOV pulses are coupled at the same time. The multiplexer MUX3holds the previous positive XOV pulse until the next XOV pulse shows up.So, the MUX3 will hold the previous positive XOV pulse during a numberof zeros between two positive XOV pulses by looping the Q output of FF6back to the D input of FF6. The present positive XOV pulse will beeventually coupled to AND 3 and positive non-alternating XOV will bedetected. If the present XOV pulse is negative, then the feedback loopon FF6 is broken and a zero level pulse is coupled to the D input of FF6and no non-alternating XOV is detected. The negative non-alternating XOVdetection circuit functions exactly the same as the positivenon-alternating XOV detection circuit. The output of OR3 is an activehigh positive/negative non-alternating XOV detection signal which iscoupled to FF8 and retimed by RLRCLK. The timing diagram of FIG. 9illustrates the details of each signal.

Performance Monitoring for MJU Circuits

As described above, in MJU networks, noisy station branches which arecausing errors, interfere with a customer's active branch, thus possiblycausing total multipoint network failure. In order to prevent a failedbranch from interfering with the data transmission on an active branch,all network control codes (bit 8=0) received on any branch must beprevented from passing through to the control channel. The OCU thatterminates a noisy station loop could transmit network control codesupstream upon detection of errors. This type of feature offering wouldachieve the error performance of two-point private line service onmultipoint customer circuits where problems have occurred with noisybranches.

The OCU that has built-in LCV detection circuits will monitor the datareceived from the loop for streaming errors. When a streaming errorcondition is detected, the OCU will send an Abnormal Station Code (ASC)(S0011110) towards the network. The ASC is one of network control codeswhich have bit 8 equal to 0. The data bits (bits 1 to 7) of a receivednetwork control byte (bit 8=0) are first changed to all ones beforefurther processing in MJU. The data bytes from the branches can then beANDed so that zero bits will always pass through to the control channel.Therefore, the data bits of errored byte detected by the OCU will beconverted to all ones, thus not interfering with active data signals.This feature inhibits data errors from corrupting an entire network andis desirable in MJU networks.

The metallic loop performance monitoring feature can be built in the OCUto monitor the metallic loop for LCVs. When too high an LCV rate isdetected, the OCU will automatically idle out the DSO channel by sendingASCs towards the network. While the OCU is in the idle state, it willcontinue to monitor the loop. When the loop performance returns tonormal (acceptable BER), the loop monitoring circuitry automaticallyreturns the OCU to an on-line state.

The loop performance monitoring circuitry will track errors in a unit oftime and measure the unit of time asynchronously to the error events.The unit of time containing one or more error events is called an"errored second", and the unit of time containing no error event iscalled as an error free second. If eight or more errored seconds in any64 second interval are detected, the loop performance monitoringcircuitry will idle out the channel by sending ASCs towards the network.In other words, the metallic loop performance feature disables servicewhen the error free seconds on the metallic loop drops below 87.5%. Thisoff-line state will continue until 30 consecutive error free seconds aredetected without the occurrence of an errors event. Then, 30 secondsafter the loop performance monitoring circuitry begins receiving validdata from the metallic loop, it will return the OCU to an on-line state.The hysteresis assures that the OCU will only return on-line when theloop problem is solved. Similar other algorithms can be created based onthe main ideas.

The loop performance monitoring is particularly valuable in multipointconfigurations where streaming data on a single branch can effectservice to customers connected to other branches of the multipointnetwork By automatically taking the noisy loop off-line when loop errorare detected, the loop performance monitoring circuitry assures thatother customers on the multipoint network will continue to receive highquality, reliable service, while the streaming branch is being repaired.

Proactive Maintenance with NIU

Referring now to FIG. 10, a DDS Smart Jack also called a DDS NetworkInterface Unit (NIU) 20 is a device which is installed at thecustomer-to-loop demarcation point which is called the Network Interface(NI). The NIU traditionally provides the telephone company with a testpoint to allow the sectionalization of troubles to either their networkor the customer's premises. To provide proactive maintenance, the loopperformance monitoring circuitry is added to the NIU. The NIU thenmonitors the incoming loop for LCVs in a manner analogous to the OCU asdescribed earlier.

When the loop performance monitoring circuitry in the NIU 20 detects an"errored second" the NIU 20 sends information to the OCU over a slowspeed data link using the 4-wire simplex path, shown in FIG. 10, as thecommunication channel. In DDS the sealing current is carried along thispath.

As shown in FIG. 10, the 4-wire simplex data link consists of two-wireloops, loops 1 and 2, in which respective transmit signals 1 and 2 arecoupled via input transformers T3 and T2, over loop lines 1 and 2 toreceive transformers T1 and T4, respectively at the NIU 20 and OCU12.

In the OCU12, a sealing current circuit is provided in which currentflows from an OCU to an NIU via center taps on the secondary windings ofT3 and T4. The secondary of T4 is grounded through R4 and the center tapof the secondary of T3 is coupled through R2 to a typical line feedvoltage of -48 volts.

The sealing current circuit path is completed in the NIU 20 by astandard terminator circuit 22 coupled between the center taps of T1 andT2.

A simple communication channel can be instrumented by pulsing thesealing current Is for 40 millisecond. As shown in FIG. 10, this pulsein the loop current may be generated by inserting an additional resistorR1 of 100 ohms into the current path at the NIU. Switch S1 is preferablya transistor switch which is triggered ON upon detection of an LCV"errored second". This pulse in the loop current is detected at the OCUthrough an AC coupled comparator circuit 24. The line feed resistorsR2,R3 at the OCU are chosen such that the sealing current is not lessthan 4 mA. As a result, because of the 100 Ohms change in the sealingcurrent path's resistance, when R1 is switched in, the 40 millisecondpulse has a magnitude of at least 400 mV, a good fraction of which iscoupled through C1 to the comparator 24. The comparator 24 compares thereceived pulse to 25 mV DC and generates an output pulse which isrecorded at the OCU as an indication of an "errored second" in thenetwork-to-customer direction.

While the loop performance monitoring circuitry in the OCU can detectloop errors only in the customer-to-network direction, the XOV and BPVfeatures described previously, allow the OCU to monitor the loopperformance in the network-to-customer direction as well. The OCUcollects and passes this information to a Proactive MaintenanceApplication circuit which uses it to declare loop alarms, bothcatastrophic, requiring immediate maintenance procedures andnon-catastrophic (early warning), for which maintenance can be scheduledas convenient (but before the circuit experiences complete outage).

Without such an NIU, the monitoring of the loop performance in thenetwork-to customer direction without a dispatch of personnel to thecustomer premises is not possible. By providing a loop performancemonitoring capability at the demarcation between network and customer'spremises the telephone company can identify troubles without dispatchingpersonnel. This proactive maintenance feature will improve the overallreliability and quality of digital networks.

In summary, a system for Loop Performance Monitoring of DDS loops isdescribed. Even though DDS loops have intentional Bipolar Violations(BPVs), a Loop Coding Violations (LCVs) detection system based onfurther processing of BPVs is described. By monitoring LCVs a local loopterminating device can determine Bit Error Rate (BER).

A system is described by which an OCU can process LCV information todetermine signal quality of the signal over the incoming local loop. Ifthe signal quality falls below a certain threshold, the OCU can cut theloop off from the DDS circuit and send control codes (ASC makes mostsense but others can be used) into the network. When an OCU is part of amultipoint circuit, this strategy has obvious beneficial effects on thedata transmission reliability of the circuit. Streaming branches nolonger take down the whole circuit.

Finally, a system is described in which a Network Interface Unit withthe Loop Performance Monitoring feature, communicates incoming LCVinformation to the OCU using slow speed signaling over the simplex path.The OCU monitors incoming LCVs as well, and thus has informationnecessary to develop bi-directional BER performance. It passes thisinformation to a Proactive Maintenance Application (PMA) whichinstruments and appropriate strategy to provide to the end user, adesired grade of service pertaining to loop reliability.

Equivalents

Those skilled in the art will recognize, or be able to ascertain usingno more than routine experimentation, many equivalents to specificembodiments of the invention described specifically herein. Suchequivalents are intended to be encompassed in the scope of the followingclaims.

The invention claimed is:
 1. Apparatus for coupling to a communicationchannel of a local loop circuit in which information is transmitted as astream of digital bits comprising:a) an LCV circuit for determining theoccurrence of loop code violations, in which a loop code violationcomprises the occurrence of an unintentional bipolar violation asdistinguished from an intentional bipolar violation; and an intentionalbipolar violation is signalled by an XOV bipolar violation sequence inwhich a violation bit V, having the same polarity as a previous bit, isintentionally inserted into the bit stream preceded by intentionalinsertion of an X bit two bits prior to the violation which may comprisea bit having a polarity or no polarity such that successive violationsalternate in polarity and intentional insertion of a zero value bitbetween the X and V bits; and b) disabling means for disabling thecommunication channel when the frequency of occurrence of loop codeviolations exceeds a predetermined rate.
 2. The apparatus of claim 1wherein, if the number of bits since the occurrence of the lastviolation bit V is an even number, a bit of opposite polarity to theprevious bit is inserted as the X bit and in the event the number ofbits since the occurrence of the last violation bit V is an odd number,the inserted X bit is a ZERO.
 3. The apparatus of claim 1 includingenabling means for enabling the communication channel when the frequencyof occurrence of loop code violations returns to a predeterminedacceptable rate.
 4. The apparatus of claim 1 in which the communicationchannel is disabled by transmitting an abnormal status code. 5.Apparatus for coupling to a communication channel in which informationis encoded as a pulse stream of alternate mark inversion bipolar pulsescomprising:a) a code violation circuit for determining the occurrence ofan unintentional bipolar sequence violation as distinguished from anintentional bipolar sequence violation wherein an intentional bipolarviolation is signalled by an XOV bipolar violation sequence in which aviolation pulse V having the same polarity as a previous pulse isinserted into the pulse stream and is preceded by insertion of an Xpulse two pulses prior to the V pulse which may have a polarity or nopolarity such that successive violation pulses alternate in polarity andinsertion of a zero value pulse between the X and V pulses; and b)disabling means for disabling the communication channel when the rate ofoccurrence of said unintentional violations exceeds a predeterminednumber.
 6. The apparatus of claim 5 wherein an unintentional bipolarviolation is determined by detecting a bipolar violation which does nothave an XOV bipolar violation sequence or an XOV bipolar violationsequence which has the same polarity as a preceding XOV bipolarviolation sequence.
 7. The apparatus of claim 1 in which the digitalbits are encoded as alternate-mark-inversion bipolar pulses andtransmitted on each of two rails of a transmit loop wherein one rail isa positive rail and the other is a negative rail which carries theinverse of the pulses on the other rail and wherein a bipolar violationoccurs when a present pulse on one rail has the same polarity as aprevious pulse on the same rail, and including a bipolar violationdetection circuit comprising:a) a first circuit for receiving thepresent pulses and outputting the present pulses from one rail; b) asecond circuit for storing the present pulses and outputting the presentpulses delayed by one pulse interval, such that the outputs of thesecond circuit are the previous pulses; c) a third circuit fordetermining the presence of pulses from the first and second circuit ofthe same polarity and indicating a bipolar violation upon suchoccurrence.
 8. A loop performance monitor for detecting the occurrenceof unintentional bipolar violations in which digital signals are encodedas alternate-mark-inversion bipolar pulses and transmitted on each oftwo rails of a transmit loop wherein one rail is a positive rail and theother is a negative rail which carries the inverse of the pulses on theother rail and wherein a bipolar violation occurs when a present pulseon one rail has the same polarity as a previous pulse on the same rail,and wherein intentional bipolar violations are indicated by an XOVseries of pulses, said monitor comprising:a) a first circuit forreceiving the present pulses and outputting the present pulses from onerail; b) a second circuit for storing the present pulses and outputtingthe present pulses delayed by one pulse interval, such that the outputsof the second circuit are the previous pulses; c) a third circuit fordetermining the presence of pulses from the first and second circuit ofthe same polarity and indicating a bipolar violation upon suchoccurrence in the event an XOV series of pulses is not present.
 9. Acommunication system for transmitting a signal indicating the receptionof loop code violations to a transmitter comprising:a) a firstcommunication loop comprising a first two wire circuit in whichcommunication signals are transmitted from a first transmitter at afirst terminal to a first receiver at a second terminal in a firstdirection by inductively coupling transmit signals to the first circuitand inductively coupling received signals from the first circuit; b) asecond communication loop comprising a second two wire circuit in whichcommunication signals are transmitted from a second transmitter at saidsecond terminal to a second receiver at said first terminal in a seconddirection by inductively coupling transmit signals to the second circuitand inductively coupling received signals from the second circuit; c) adirect current path running from the first circuit to the secondcircuit; d) a direct circuit power source coupled across the directcurrent path at the first terminal; e) a switch in the direct currentpath at the second terminal generating a current pulse indicating thereception of loop code violations; and f) a detector coupled to thedirect current path at the first terminal for detecting the occurrenceof the pulse.
 10. The system of claim 9 wherein the inductive couplingis provided by a pair of transformers at each terminal, each transformerhaving primary and secondary windings and the direct current path iscompleted through taps on said windings.
 11. The system of claim 10 inwhich the pulse is formed by inserting an impedance into the directcurrent path.
 12. The system of claim 10 in which the pulse is detectedby a voltage comparator circuit.
 13. A method of communication in whichinformation is transmitted as a stream of digital bits comprising thesteps of:a) determining the occurrence of loop code violations, whereina loop code violation comprises the occurrence of an unintentionalbipolar violation as distinguished from an intentional bipolarviolations and wherein an intentional bipolar violation is signalled byan XOV bipolar violation sequence in which a violation bit V, having thesame polarity as a previous bit, is intentionally inserted into the bitstream preceded by intentional insertion of an X bit two bits prior tothe violation which may comprise a bit having a polarity or not polaritysuch that successive violations alternate in polarity and intentionalinsertion of a zero value bit between the X and V bits; and b) disablingthe communication when the frequency of occurrence of loop codeviolations exceeds a predetermined rate.
 14. The method of claim 13wherein, if the number of bits since the occurrence of the lastviolation bit V is an even number, a bit of opposite polarity to theprevious bit is inserted as the X bit and in the event the number ofbits since the occurrence of the last violation bit V is an odd number,the inserted X bit is a ZERO.
 15. The method of claim 13 including thestep of enabling the communication channel when the frequency ofoccurrence of loop code violations returns to a predetermined acceptablerate.
 16. The method of claim 13 in which the communication channel isdisabled by transmitting an abnormal status code.
 17. A method ofcommunication in which information is encoded as a pulse stream ofalternate mark inversion bipolar pulses comprising the steps of:a)determining the occurrence of an unintentional bipolar sequenceviolation as distinguished from an international bipolar sequenceviolation, wherein an intentional bipolar violation is signalled by anXOV bipolar violation sequence in which a violation pulse V having thesame polarity as a previous pulse is inserted into the pulse stream andis preceded by insertion of an X pulse two pulses prior to the V pulsewhich may have a polarity or no polarity such that successive violationpulses alternate in polarity and intentional insertion of a zero valuepulse between the X and V pulses; and b) disabling communication whenthe rate of said unintentional violations exceeds a predeterminednumber.
 18. The method of claim 17 wherein an unintentional bipolarviolation is determined by detecting a bipolar violation which does nothave an XOV bipolar violation sequence or an XOV bipolar violationsequence which has the same polarity as a preceding XOV bipolarviolation sequence.
 19. The method of claim 18 in which the digital bitsare encoded as alternate-mark-inversion bipolar pulses and transmittedon each of two rails of a transmit loop wherein one rail is a positiverail and the other is a negative rail which carries the inverse of thepulses on the other rail and wherein a bipolar violation occurs when apresent pulse on one rail has the same polarity as a previous pulse onthe same rail, and including the steps of:a) receiving the presentpulses and outputting the present pulses from one rail; b) storing thepresent pulses and outputting the present pulses delayed by one pulseinterval, such that the outputs of the second circuit are the previouspulses; c) determining the presence of pulses from the first and secondcircuit of the same polarity and indicating a bipolar violation uponsuch occurrence.
 20. A method for transmitting a signal indicating thereception of loop code violations to a transmitter comprising the stepsof:a) communicating signals from a first transmitter at a first terminalto a first receiver at a second terminal in a first direction byinductively coupling transmit signals to a first circuit and inductivelycoupling received signals from the first circuit; b) communicatingsignals from a second transmitter at said second terminal to a secondreceiver at said first terminal in a second direction by inductivelycoupling transmit signals to a second circuit and inductively couplingreceived signals from the second circuit; c) generating a direct currentpulse indicating an loop code violation in a current path formed throughthe first and second circuits; and d) detecting the occurrence of thepulse.